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The T-junction has been extensively employed in nuclear power plant systems. To ensure the normal operation of nuclear power plant, the onset point of T-junction liquid entrainment requires in-depth studies. Existing studies have been conducted on the onset point of liquid entrainment, whereas most of them are for small branch pipes. This study primarily discussed the onset law of large branch pipe. Moreover, the effects of branch angle and size on the onset point of liquid entrainment were analyzed by comparing it with other branch pipe sizes in this study. This study demonstrated that the effect of branch angle on the onset point of entrainment complies with that of other sizes, i.e., because that the decrease in the inertial force in the vertical direction acting on the liquid phase by the gas phase with the decrease in the angle, hence, the onset requires larger Fr g under the identical h b with the decrease in the angle between the branch pipe and the horizontal direction. Moreover, by comparing the onset point law of liquid entrainment exhibiting different branch pipe sizes, the effect of branch pipe size on the onset was reported in this study that the smaller branch pipe requires the greater gas velocity in the presence of onset. We found that there is different flow resistance of different sizes of branch pipe, small branch has larger flow resistance, in order to remedy the flow resistance, the gas dynamic head decreases, thereby causing the Bernoulli effect weaker; the onset of liquid entrainment requires larger V 3g .

The influence of the size and the angle of the branch on the onset of gas entrainment is explored in the present study. Since previous studies were performed on small-sized branches and the angles of the branches were specific (0 • or −90 • ), it is difficult to apply them to arbitrary-angled branches. So we conducted a series of experiments in a different direction of −30 • , −45 • , −60 • , and −90 • angles with the main pipe of 80-mm diameter and a branch of 31-mm diameter (T-tube). A new correlation is developed and can predict the most experimental data well within the error range of ±20%. And the new correlation can predict the onset of gas entrainment at arbitrary-angled branch and is not constrained by the angle of branches; that is, it has good versatility. We also gained some meaningful conclusions by analyzing experimental data and the entrainment phenomenon: gas is entrained by a vortex flow, and the size of the branch and the angle of the branch have little effect on the form of entrainment at a given h b /d. The critical Froude number (Fr L ) gradually increases as the angle of the branch changes from −90 • to −30 • . When this is compared with previous models, it can be found that the correlation of the onset of gas entrainment based on small-sized branches is not suitable for large-scale branches. But we have defined a new critical liquid level (h * b ) to successfully expand the scope of application of the correlation.Keywords: T-tube, onset of gas entrainment, size of branch, angle of branch, the onset of phenomenon

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